Solid lubrication offers many benefits over conventional oil-based hydrodynamic and boundary lubrication. Solid lubrication systems are generally more compact and less costly than oil lubricated systems since pumps, lines, filters and reservoirs are usually required in oil lubricated systems. Greases can contaminate the product of the system being lubricated, making it undesirable for food processing and both grease and oil outgas in vacuum precluding their use in space applications. One of the primary goals of a solid lubricant is obtaining low friction.
Polytetrafluoroethylene (PTFE) is known by the trade name TEFLON®. PTFE is well known as a low friction material and has thus received much attention for use as a solid lubricant. It also has other desirable properties including, high melting temperature, chemical inertness, biocompatibility, low outgassing and low water absorption. However, PTFE wears much more rapidly than most other polymers preventing its use as a bearing material in many cases.
It is known that copper and graphite greatly improve the life of PTFE when used as fillers. Glass fibers and micron sized ceramics have also been shown to improve wear resistance of PTFE. These fillers are thought to reduce wear because they preferentially support the load. Briscoe et al (Briscoe, B. J., L. H. Yao, et al. (1986). “The Friction and Wear of Poly(Tetrafluoroethylene)-Poly(Etheretherketone) Composites—an Initial Appraisal of the Optimum Composition.” Wear 108(4): 357-374) disclose a PEEK/PTFE polymer/polymer composite, comprising a plurality of discrete PTFE particles in a polyether ether ketone (PEEK) matrix. PEEK has low wear and high friction and PTFE has high wear and low friction. Briscoe et al. found a disproportionate drop in microhardness, compressive strength and Young's modulus of the PEEK matrix with the addition of small amounts of PTFE, indicative of poor adhesion at the particle-matrix interface. The wear rate of the composite was reported to increase linearly from unfilled PEEK to 3 times the wear rate of unfilled PEEK for the 70 wt % PTFE composite. Wear was reported to be accelerated beyond 70 wt % PTFE. Briscoe et al. concluded that the 10 wt % PTFE composite is optimal.